1. Field of the Invention
This invention relates to the use of anaerobic digesters to generate fuel from biomass through biological processes.
2. Prior Art
Animal manures, spoiled animal feed, food wastes and other forms of biomass can be used for producing a biogas rich in methane. This biogas can be produced using anaerobic digesters and is useful as a fuel for generators and other applications. Anaerobic digesters can be made according to a multitude of different architectures that are typically tailored to the needs of a specific user.
From the 1930s onwards digesters have been built through national programs in India, China and other Southeast Asian countries to supply cooking fuels for small family farms. More recently, developing countries have developed a strong interest in using digester technology to address the need for inexpensive fuels for heating and electrical generation. Farms in the Europe and North America have recognized that digesters offer an attractive way of processing animal wastes to reduce associated odors and/or fuel generators.
Over the past several decades, this technology has proven itself feasible on large farm operations (LFOs) and has repeatedly failed on small and medium-sized farms. This is due, in large part, to the expense of building large structures to hold the feedstock (generally manure) for the length of time required for biological processes to produce biogas. Regardless of the increasing variety of available architectures, most of these designs incorporate disadvantages that hinder their use on small and mid-sized farms:
(a) Most digesters are designed to be economical only at a large scale. Their construction requires the employment of engineers to resize and adapt the specific design to any particular construction site. Specialty contractors are generally required to build the digester due to design requirements for poured and reinforced concrete and other heavy equipment and/or materials specified in these designs.
(b) Their operation and maintenance requires more time and expense than small or mid-sized farmers can afford. Most digesters lack specific mechanisms that simplify the periodic removal of sediments that will naturally accumulate within the digester.
(c) Most digesters require a large array of pumps and other electrical and mechanical devices that must run continuously for effective operation. The inevitable failure of these devices in the farm's harsh environment may greatly inconvenience farmers whose operations already call for heavy maintenance schedules. Moreover, these devices may use a significant fraction of the energy value of the biogas, making digesters feasible only on large-scale farms.
(d) Most plug flow designs appropriate for manures have flexible membrane covers that have a limited lifetime and failure prone systems for attachment, leading to biogas leaks. The cover must be removed to allow periodic cleanouts, and the difficulty of this process leaves the owner/operator balancing this cost against the potential decrease in performance due to accumulated sediments.
(e) Most digesters are designed to store a useful amount of biogas within the digester itself. Most digesters, however, store this biogas at low pressure that may require that gas piping be of larger diameter or that gas use be restricted to within a short distance of the digester. Digesters that are mechanically pressurized typically require the use of compressors, pumps or compressed gases. Additionally, the pressurized digester itself becomes a pressure vessel that requires elaborate valving to enable feedstock flow.
(f) Most digesters that handle high levels of solids fall into the low range of efficiency with regard to their ability to convert volatile solids in the feedstock to methane. Digesters of the ‘plug flow’ variety may be vulnerable to low efficiency operation, due to continuous loss of microbes that dwell in the feedstock. High efficiency digesters typically employ a ‘fixed-film’ that supports development of a microbial biofilm, where digestion of feedstock is thought to occur as nutrients in the feedstock permeate this biofilm. These fixed-film digesters, however, require that the feedstock contain less than 8% solids, in order to avoid destroying the biofilm. This upfront removal of solids causes a significant part of the odor-causing and digestible material to be excluded from receiving the benefits of anaerobic digestion.
(g) Farmers may have to finance the purchase of even a small-scale digester. Lending institutions may be less likely to give loans for fixed assets. Most digesters, being essentially custom-built using poured concrete and permanent structures must be considered to be fixed assets.